Air compressors are widely used for the compression of air for use in blow moulding processes for forming PET bottles or the like. In the flowing, reference will be made to blow moulding of PET (polyethylene terephthalate) products, however, it should be understood the invention also relates to blow moulding of products from other materials similar to PET. In most manufacturing facilities, compressed air is required at different and sometimes variable, pressure levels. Low compressed air (eg 6 to 10 bars) may be needed for pre-form inflation, factory automation and other general service compressed air requirements, and high pressure compressed air (eg 30 to 40 bars) may be required for bottle blowing to a completed standard. Commonly it is the current practice to use separate compressors to achieve the foregoing requirements, ie a dedicated low pressure compressor and a dedicated high pressure compressor. In some installations, low pressure air can be supplied from a single high pressure compressor via a pressure reducing valve. Use of two machines complicates installation and maintenance and supply of low pressure compressed air via a pressure reducing valve from a high pressure compressor involves significant energy losses. Thus, either of these alternatives has its disadvantages.
A further complication is that compressed air usage in blow moulding installations varies considerably depending on the number of bottles being blown per hour and the size of the bottles being blown. Furthermore, in this industry there are strong seasonal demands in the numbers of blown bottles required to be produced.
One option is to take a supply of low pressure compressed air from an intermediate stage of a multistage compressor. For example, if the low pressure compressed air desired is 10 bars, it may be extracted from the second stage of a three stage machine which typically might be at 12 bars. A pressure reducing valve may be required to regulate the pressure back to 10 bars and although there is an energy loss through this pressure reducing valve, the loss is much smaller than reducing the pressure to 10 bars from higher pressure such as at 40 bars.
Another possible option might be if the speed of the compressor could be adjusted by means of a variable speed drive (VSD) from say 100% capacity to 20% capacity. If this were possible, it might provide a reasonable solution, however, the application of VSD to piston compressors is difficult due to balance, lubrication and valve requirements. Typically, such piston compressors should not be slowed down to less than 70 to 80% of their full load speed. Furthermore, VSD systems tend to be expensive and may be unreliable. As a result, it is believed desirable to run piston compressors at a fixed speed.
If the first stage of a multistage piston compressor is unloaded partially (so called step unloading, whereby one side of a double acting compressor cylinder is unloaded), the interstage pressures of each subsequent stage (from where the extracted air might be taken) falls. This causes problems when either the compression ratio in the final stage exceeds a safe limit, or the interstage pressure is lower than the desired pressure.
There is a need therefore to be able to provide compressed air at two desired pressure levels from a single compressor, conveniently, a multistage piston compressor.